A large group of minerals in which boron is chemically bonded to oxygen. Boron is a fairly rare element. However, because of its chemical character, it is very susceptible to fractionation in Earth processes and can become concentrated to a degree not found in other elements of similar abundance. Boron is symbolised B, has the atomic number 5, and has the ground-state electronic structure (He)2s²2p¹. The very high ionisation potentials for boron mean that the total energy required to produce the B³⁺ ion is greater than the compensating structure energy of the resulting ionic solid, and hence bond formation involves covalent (rather than ionic) mechanisms. However, boron has only three electrons to contribute to covalent bonding involving four orbitals, (s, p_x, p_y, p_z). This results in boron being a strong electron-pair acceptor (that is, a strong Lewis acid) with a very high affinity for oxygen. The structural chemistry of boron and silicon (Si), when associated with oxygen (O), is quite similar. The BO₃, BO₄, and SiO₄ groups have a marked tendency to polymerize in the solid state, and this aspect of their behaviour gives rise to the structural complexity of both groups. However, subtle differences in chemical bonding do give rise to differences in the character of this polymerization, particularly when H₂O is also involved. These differences result in the very different properties of the resultant minerals and their very different behaviour in Earth processes.